[BC] 12-pulse power supplies

[RichardBJohnson at comcast.net]

Many have already replied. The fact is that it is not obvious how one could obtain 12 pulses from 3-phase, 60 Hz. A full-wave rectifier will only give you 60 * 2 = 120Hz for a single phase. You have three-phases so, intuitively one would have 120 * 3 = 360 Hz as the ripple component.

Suppose we connect two power supplies in series, but we rotate the phase of one of the three-phase inputs by 60 degrees so that the ripple from one power supply fits in-between the ripple of the other power supply. We now have a 12-pulse power supply. So far-so good, but how do we rotate the phase? In the "olden days" engineers designed a transformer with a so-called "interphase" winding, also called "zigzag," that would select the right amount of voltage from each phase as:

Select the right amount of voltage from phase-A, and the right amount of voltage from phase-B and subtract (or add) it from phase-C.

Select the right amount of voltage from phase-B, and the right amount of voltage from phase-C and subtract (or add) it from phase-A.

Select the right amount of voltage from phase-C, and the right amount of voltage from phase-A and subtract (or add) it from phase-B.

This would give you a three-phase circuit in which the phase was advanced (or retarded) by 60 degrees. In the stated example, "the right amount" can be calculated using a phasor diagram or directly using some math (for which my book is at home).

As I previously stated, this was used for many years in the broadcast industry. Such a transformer was very expensive because it contained six special tapped windings on the secondary.

Now let us look at what happens to a delta connection fed to a full-wave rectifier. Note that the three rectifiers conduct in groups around the unit-circle (phasor diagram), and supply pulses of current to the load that are essentially in-phase with the transformer primary voltage if the transformer has low leakage reactance.

The wye connection is different, even though the phase-to-phase voltage is the same as that of the delta. When the wye circuit feeds current pulses to the load, the current flows through the reactance of the winding, sort of like a choke-input filter. This causes the current to lag the transformer primary voltage. If the load current is very low, the reactance is not significant so the phase-lag is small. If the current is very high, i.e., a short-circuit, the current will lag the primary voltage by 90 degrees (a pure reactance).

Therefore, depending upon the load-current the current-pulses from the wye-connected secondary can fit in-between the current-pulses from the delta-connected secondary.

Enter Kirchoff! His law claims that all series-connected currents are the same. If we connect both rectifier circuits in series, forcing the same load-current through both rectifier circuits, the current pulses are forced to the same magnitude. The only way for this to happen is if the phase difference adjusts itself to 60 degrees.

This kind of power supply has many redeeming qualities, but two shortcomings.

First, the power supply needs a minimum load current to provide 12-pulse operation.

Second and this can be a "getcha;" Many transmitters have step-start connecting the primary of the transformer to wye for low power, then transfers to a delta-connected primary for high power. The manufacture connects that step-start relay. If one happens to switch the phases on the transformer side of that relay, so the ABC phase sequence, when delta-connected, is not the exact same ABC phase sequence of the delta-connected secondary, one could end up with any phase unbalance in the whole plant being "corrected" by that HV power supply transformer NotGood(tm).

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Dave Dunsmoor" <mrfixit at min.midco.net>

> Since 12-pulse (or 12-phase) transformers were expensive, ...

    Richard (or anyone else who would care to reply), I've asked this
question before, and now am going to have to admit to some forgetfulness:
how exactly does this 12-phase business work? Where do you get the
nomenclature "12 pulse" (or phase) from a 3 phase incoming main AC, and how
does combining 2 secondaries constitute something that can be considered to
be 12 phases?